Anyone who regularly works with gemstone chip bead will already be aware of the vast variations in size shape and colour. I can only give you are rough guide, but I hope it will help!
This guide applies to the chips that I (destiny-uk)sell, and my own experience, other people may get them in different sizes.
The smallest always seem to be...Citrine, Peridot, Labradorite, Blue & Goldstone, Tourmaline,(which can be as small as 3mm, but on average would be 4 to 5mm.
The largest ones are usually Moss Agate, Black 'Onyx', Mother of Pearl which would measure between 7 & 11mm
Most of the others are middle sized which would cover from 5 to 8mm
So if for example you wanted to make a braclet from light and dark green stones , and you ordered Peridot for the light and Moss Agate for the dark, the Agate would look HUGE nest to the Peridots! If you were to ask me, I would recommend Jade and Aventurine, which both be roughly the same size, so always best to check first to avaoid dissapointment.
Another example would be, if you wanted to make delecate looking necklace with grey stone, and you chose grey Moonstone, you may be dissapointed with the result, as the chips are on the biggish side. Here I would have recommended Labradorite, which are very dainty looking, and really twinkly.